A combination of neuroanatomical techniques will be concentrated on analyzing the detailed neuronal morphology and patterns of connectivity in an anatomically circumscribed region of the central nervous system, namely, that region in the spinal cord of the cat - the most medial portion of the seventh lumbar segment of the dorsal horn - in which physiological study has revealed the location of interneurons mediating the plantar cushion (PC) reflex. The neuroanatomy should complement the physiology in providing detailed evidence of the anatomical substrate for transmission of this reflex, and for modulation in its transmission. Because the circuitry of only a small fraction of the spinal gray matter is primarily involved in PC reflex transmission, it is possible that this anatomical analysis can be more successfl than attempts to analyze the circuitry of the dorsal horn as a whole. Among the anatomical techniques to be employed are: modified Golgi stains, degeneration techniques, low-power electron micrographs, and stains to be uued in conjunction with neurophysiological recording, including horseradish peroxidase and intracellular fluorescent dyes. Once the adult morphology of these regions has been sufficiently well-defined, it may be possible to detect structural correlates of physiological changes in PC reflex function occurring in the course of development.